Method and arrangement to rapidly lower the rotary spped of a centrifugal separator

ABSTRACT

In a method of rapidly lowering the rotary speed of a rotating rotor body in a centrifugal separator the lowering of the rotary speed is initiated, when a disturbing unbalance or vibrations have occurred. A space between the rotor body and a stationary casing which surrounds the rotor body is supplied with foam or with an aerosol of a liquid in a gas, when unbalance or vibrations over a certain level have occurred. The rotor body is retarded by the friction, which is obtained between the rotor body, the foam or aerosol and the casing. In an arrangement to lower the rotary speed of a rotating rotor body in a centrifugal separator the lowering is initiated when a disturbing unbalance or disturbing vibrations has occurred. Means are arranged at a certain unbalance or certain vibration level to activate a foam generating device or a device that creates an aerosol for the purpose of filling a space between the rotor body and a casing which surrounds the same with foam or aerosol.

[0001] The present invention relates to a method to rapidly lower the rotary speed of a rotating rotor body in a centrifugal separator. Lowering of the rotary speed is initiated when a disturbing unbalance or disturbing vibrations have occurred. According to the invention also an arrangement to achieve this effect is suggested.

[0002] Centrifugal separators are usually provided with different kinds of braking devices. The braking devices may consist of a mechanical brake acting on the rotor body or on the driving shaft that sets the rotor body in motion. The rotor body may also be retarded in that the motor, which directly or indirectly makes the driving shaft rotate, is switched such that it works as a brake instead. It may take between 5 to 10 minutes to make the rotor body stop completely in a planned shutdown, since the rotating mass of the rotor body is large.

[0003] A centrifugal separator with a liquid brake is shown in DE 178 26 47. The liquid brake comprises a ring with a U-formed cross section arranged on a stationary stand. When the centrifugal separator shall be retarded the U-formed ring is filled with liquid. A braking disc, which is arranged on the rotor body, is arranged to stick into the U-formed ring. When the ring contains liquid the friction between the braking disc and the liquid creates a retarding force. In the patent specification there is also described an arrangement where the disc is stationary, while the U-ring is arranged on the rotating drum.

[0004] As a security measure centrifugal separators intended for certain applications are provided with vibration alarms, which give a signal if a disturbing unbalance or dangerous vibrations occur. When an alarm has been triggered one tries to retard the body with the braking device. If the unbalance has occurred during a complete or a partial discharge of the collected sludge from the centrifugal separator, one tries to reduce the unbalance by filling the rotor body with water and closing the flow of liquid entering the centrifugal separator. In both these cases it will be a certain time before the rotary speed is so low that the unbalance does not cause any damages.

[0005] The present invention aims at creating possibility of, in a rapid and reliable manner, lowering the rotary speed of a rotating rotor body in a centrifugal separator. This purpose is according to the invention achieved in that a space between the rotor body and a casing surrounding the same is supplied with foam or an aerosol of liquid in a gas, when unbalance or vibrations over a certain level has occurred. The rotor body is retarded by the friction, which is obtained between the rotor body, the foam or aerosol and the casing. The intention of the retardation may either be to make the rotor body stop completely or to retard it such that the rotor body rapidly passes the interval of numbers of revolutions, where disturbing vibrations or unbalance may damage the centrifugal separator. It has surprisingly been found that a supply of foam or an aerosol of liquid in gas has a considerable retarding influence. The foam consists of gas or air bubbles separated by thin liquid lamellas. The density of the foam depends on one hand on the density of the liquid, which forms the foam and on the other on the relation between the amount of liquid and gas or air. The aerosol consists of small liquid drops distributed in a gas. Also here the density depends on the density of the liquid and on the relation between the amount of liquid and gas. The retarding effect is directly proportional to the density of the medium that surrounds the rotor body.

[0006] With advantage the supply of foam or aerosol is initiated by the radial displacement of the rotor body that appears during unbalance. Alternatively the supply may be initiated by a vibration meter, which gives a signal when the vibrations exceed a certain level.

[0007] In order to avoid damages it is important that a retarding force rapidly has effect. The foam or the aerosol is therefore supplied during a short period suitably less than 30 seconds.

[0008] The amount of foam or aerosol, which must be supplied, is adapted to the size of the rotor body. For a small centrifugal separator only some litres may be needed, for a large centrifugal separator the volume may be several tens of litres.

[0009] It may be of a great advantage that the water in the foam or in the aerosol, which is vaporised by the rise in temperature due to friction, lowers the temperature of the rotor body during the retardation.

[0010] With advantage a foam is made use of which is of such a kind that it has fire extinguishing properties. In many applications inflammable liquids are treated. It may also be so that the centrifugal separator is used under conditions, where a fire may be fatal.

[0011] An arrangement to lower the rotary speed of a rotating rotor body in a centrifugal separator, which lowering is initiated when a disturbing unbalance or disturbing vibrations have occurred, comprises means arranged at a certain unbalance or at a certain level of vibrations to activate a foam generating device or a device which creates an aerosol for the purpose of filling a space between the rotor body and a casing surrounding the same with foam or aerosol.

[0012] If a disturbing unbalance, which results in a radial displacement of the rotor body, is caused by an uneven collection of sludge, said means may suitably be arranged to activate the foam generating device, which is connected to the space between the rotor body and the surrounding casing in order to fill the space with foam.

[0013] Said means to activate the foam generating device consists with advantage of a part projecting from the rotor body, which part at unbalance mechanically actuates a closing device on a pipe, which is connected to the foam generating device. The foam generating device suitably comprises a container filled with foam creating liquid and gas.

[0014] The device, which supplies aerosol to the space, comprises a container with water under high pressure and a nozzle, which when the container is opened creates a water mist with very small droplets.

[0015] According to the invention one achieves a retarding force by filling the space between the rotating rotor body and the casing with foam or with aerosol. A rotating rotor body in a centrifugal separator rotating at full speed represents a considerable amount of kinetic energy. The size of this force varies of course with the size of the rotor body. Depending on the desired time for retardation a suitable density of the foam or aerosol is chosen. Should the rotor body be retarded by supplying solely water to the space, the stress on the surrounding casing would be so large that there are risks for damages. With only air in the space the retarding force would be so small that the time from full rotation to immobility could amount to 30-90 minutes. The retardation or the retarding moment is directly proportional to the density of the medium surrounding the rotor body.

[0016] Suitably one uses foam with a relatively large mechanical stability, which does not disintegrate directly or is thrown against the casing such that a thin air gap is formed nearest to the rotor body.

[0017] If the rotor body is retarded with an aerosol the liquid drops should be so small that the lifting forces are of the same magnitude as the force of gravity so that the drops remain in the aerosol during a certain time after the supply to the space.

[0018] A kind of foam that is suitable to use for this purpose is fire extinguishing foams. These foams are formed by surfactants, for example fluorinated surface-active substances, glycol, tensides and polysaccharides, which are mixed with water in order form a premix. In order to create the foam air or gas is supplied under pressure. Depending on the mixing relation between premix and air or gas foams with different densities are obtained. The shape of the nozzle, where the foam is created, determines the degree of expansion, i.e. the relation between liquid and gas or air in the foam. A high degree of expansion gives a lighter foam, while a larger amount of liquid gives a heavier foam.

[0019] Fire extinguishing equipment for creating water mist may also be used for creating a suitable aerosol. Such equipment comprises containers with a propellent gas of a suitable kind. The water mist is created in that the water is brought to pass special nozzles, which divide the water into very small droplets. Such a water mist comprises water droplets in the order of magnitude of 50-200 μm.

[0020] The attached drawing shows how the time to retard a rotor body depends on the size of the rotor body and on the volume ratio of water in the medium, which surrounds the rotor body. The graph with number 1 represents the rotor, which is smallest, while the graph given number 9 relates to a rotor, which is five times larger. As may be seen also a small amount of water may diminish the retarding time in a considerable degree. If the rotary speed is lowered to below 800 revolutions/min the risk for damages may be reduced considerably. In order to achieve this goal it is needed that the volume ratio of water must be in the interval 0,4-0,7%.

[0021] Below there is shown the result of a trial of retarding the rotor body in a centrifugal separator with foam.

[0022] In the trials an extinguishing agent was utilised which is used in foam extinguisher found on the market. The extinguishing agent was mixed with water in different proportions and the mixture was put under pressure in a pressure vessel. The pressure vessel was in its upper part provided with a pipe, which was connected to the space between the rotor body and a surrounding casing. An expansion nozzle was arranged in the part of the pipe turned towards the centrifugal separator at the inlet to the said space.

[0023] During the trials there was used a centrifugal separator with a rotor radius of 300 mm which through all the trials rotated with the same number of revolutions, 10400 rpm, when the supply of foam was started.

[0024] During the trials the number of revolutions and the retardation time was measured. The first trial was made with only a small amount of foam. The intention was to control that the rotor body was not influenced in such a manner that it turned unstable during the retardation.

[0025] The foam generating liquid is available in the form of a concentrate, which is mixed with a suitable amount of water. In trial No. 1 the amount of liquid used to produce foam was 4 litres containing 10% concentrate. The liquid was transferred to a container and was pressurised. During the trials described below the pressure in the vessel was 20 bar. When the container was opened in that the closing device was punctured the foam creation was started and went on during 20-30 seconds. TABLE Trial No. 1 2 3 4 5 Amount liquid in lit. 4 6 8 6 4 Concentrate in % 10 10 15 20 20 Rotation speed when 6000 0 0 0 20 trial was stopped Stopping time — 60 60 — Seconds

[0026] The retarding effect upon the rotor body stopped at 500 r/min in trial No.2.

[0027] Judging from these trials 6-8 litres of liquid containing at least 10% foaming agent was needed for complete retardation of the centrifugal separator that was tested. When the centrifugal separator had come to a halt the space between the rotor body and the casing was filled with foam that could be rinsed away. 

1. Method to rapidly lower the rotary speed of a rotating rotor body in a centrifugal separator, at which the lowering is initiated when a disturbing unbalance or vibrations have occurred, characterized in that a space between the rotor body and a stationary casing, which surrounds the rotor body, is supplied with foam or an aerosol of liquid in a gas, when unbalance or vibrations over a certain level has occurred, at which the rotor body is retarded by the friction, which is obtained between the rotor body, the foam or the aerosol and the casing.
 2. Method according to claim 1, characterized in that the supply of foam or aerosol is initiated by the radial displacement of the rotor body caused by unbalance.
 3. Method according to claim 1, characterized in that the supply of foam or aerosol is initiated by a vibration sensor, which gives a signal when the vibrations exceed a certain level.
 4. Method according to any of the preceding claims, characterized in that foam or aerosol is supplied to the space during a time period below 30 seconds.
 5. Method according to any of the preceding claims, characterized in that foam or aerosol is supplied in a volume which depends on the size of the rotor body.
 6. Method according to any of the preceding claims, characterized in that the water in the foam or in the aerosol, which is vaporised lowers the temperature of the rotor body.
 7. Method according to any of the preceding claims, characterized in that the supplied foam is of such a kind that it has fire extinguishing properties.
 8. Arrangement to lower the rotary speed of a rotating rotor body in a centrifugal separator, at which the lowering of the rotary speed is initiated when disturbing unbalance or disturbing vibrations have occurred, characterized in that means are arranged at a certain unbalance or at a certain level of vibrations to activate a foam generating device or a device that creates an aerosol for the purpose of filling a space between the rotor body and a casing, which surrounds the same, with foam or aerosol.
 9. Arrangement according to claim 8, characterized in that said means are arranged, at a certain unbalance depending on a radial displacement of the rotor body caused by an uneven collection of sludge, to activate the foam generating device, which is connected to a space between the rotor body and a surrounding casing in order to fill the space with foam.
 10. Arrangement according to claim 9, characterized in that the means to activate the foam generating device consists of a part projecting out from the rotor body, which at unbalance, mechanically actuates a closing device on a pipe connecting the foam generating device to the centrifugal separator.
 11. Arrangement according to claim 10, characterized in that the foam generating device comprises a container filled with foam creating liquid.
 12. Arrangement according to claim 8, characterized in that the device that supplies aerosol to the space comprises a container with water under a high pressure and a nozzle, which, when the container has been opened, creates a water mist with very small water droplets. 